The present invention relates to the field of antennas and is more particularly concerned with steerable antenna systems for transmitting and/or receiving electromagnetic signals.
It is well known in the art to use steerable (or tracking) antenna systems to communicate with a relatively moving target. Especially in the aerospace industry, such steerable antennas preferably need to have a high gain, low mass, and a high reliability. One way to achieve such an antenna system is to provide a fixed feed source, thereby eliminating performance degradations otherwise associated with a moving feed source. These degradations include losses due to mechanical rotary joints, RF cable connectors; flexible waveguides, long-length RF cables associated with cable wrap units mounted on rotary actuators or the like.
Also, such steerable/tracking antennas should be designed such as to avoid a so-called keyhole effect, which is a physical limitation due to the orientation of the antenna rotation axis and caused by a limited motion range of an actuator or the like. This effect forces the antenna to momentarily disrupt communication when reaching the physical limitation to allow for the actuators to reposition before resuming the steering, thereby seriously affecting the communication capabilities of the entire antenna system.
U.S. Pat. No. 6,043,788 granted on Mar. 28, 2000 to Seavey discloses tracking antenna system that is substantially robust and includes a large quantity of moving components that reduce the overall reliability of the system. Also, the steering angle range of the system is limited by the fixed angle between the boresite of the offset paraboloidal reflector and the kappa axis determined by the distance between the offset ellipsoidal subreflector and the offset paraboloidal reflector; a wide range requiring a large distance there between, resulting in a large antenna system that would not be practical especially for spaceborne applications.
It is therefore a general object of the present invention to provide a steerable antenna system with a fixed feed source that obviates the above-noted disadvantages.
Another object of the present invention is to provide a steerable antenna system with a fixed feed source that enables beam steering over a full spherical (4xcfx80 steradians) angular range with minimum blockage from its own structure, whenever allowed by the supporting platform.
A further object of the present invention is to provide a steerable antenna system with a fixed feed source that enables tracking of a remote station without any keyhole effect over any hemispherical coverage (2xcfx80 steradians).
Yet another object of the present invention is to provide a steerable antenna system with a fixed feed source having a high gain, an excellent polarization purity and/or low sidelobes.
Still another object of the present invention is to provide a steerable antenna system with fixed feed source having simple actuation devices as well as locations of the same.
Another object of the present invention is to provide a fixed-feed source steerable antenna system that can be so positioned with a first actuator as to enable tracking of a same orbiting remote station using only a second actuator when the orbit passes in proximity to the zenith of the system location.
A further object of the present invention is to provide a fixed-feed source steerable antenna system that can be mounted on either an orbiting spacecraft or a fixed station and track a ground station or an orbiting spacecraft respectively, or be mounted on a spacecraft and track another spacecraft.
Other objects and advantages of the present invention will become apparent from a careful reading of the detailed description provided herein, within appropriate reference to the accompanying drawings.
According to the present invention, there is provided a steerable antenna system for transmitting and/or receiving an electromagnetic signal to/from a target relatively moving therearound, said system comprises:
a hyperbolic subreflector secured to a frame rotatably mounted on a support structure;
a feed source located at a first focus of the subreflector for transmitting and receiving the signal to and from the same respectively, the feed source being secured to the support structure and having a source axis pointing at the subreflector;
a parabolic reflector having a focus in common with a second focus of the subreflector for transferring the signal from and to the same respectively; the parabolic reflector being secured to the frame and having a beam axis;
a planar reflector having a normal axis intersecting the beam axis with a predetermined angle for transferring the signal from and to the parabolic reflector respectively, the planar reflector being rotatably mounted on the frame for transferring the signal to and from the target;
a first rotating member rotating the frame about the source axis; and
a second rotating member rotating the planar reflector about the beam axis, thereby having the system to steer at the target.
Preferably, the system includes a controller controlling rotation of the first and the second rotating members; thereby controlling the system to steer at the target.
Preferably, the first and the second rotating members allow for the antenna system to steer at the target anywhere within a full spherical angular range.
Preferably, the source axis and the beam axis are co-planar, thereby defining an antenna plane rotating about the source axis.
Preferably, the beam axis is perpendicular to the source axis.
Preferably, the planar reflector is of a generally elliptical shape to provide circular projections along the beam axis and a direction of the target.
Preferably, the predetermined angle is a 45-degree angle, thereby reflecting the signal from the parabolic reflector within a signal plane perpendicular to the beam axis.
Preferably, the feed source including a horn and the support structure are mounted on a generally planar platform substantially parallel to the source axis.
Alternatively, the feed source including a horn and the support structure are mounted on a generally planar platform substantially perpendicular to the source axis.
Preferably, the controller includes a first and a second encoders mounted on the first and the second rotating members respectively for providing feedback of a position of the respective rotating member to the controller.
Preferably, the feed source is a dual frequency dual circular polarization feed source.
Preferably, the controller simultaneously drives the first and the second rotating members to have the antenna system steering in a desired direction.
Preferably, the controller provides commands to the first and the second rotating members that automatically steer at the moving target.
Preferably, the first and the second rotating members are a first and a second stepper motors respectively.
Preferably, the frame minimizes blockage and interference of the signal.
Preferably, the support structure is mounted on a spacecraft planet facing panel and the target is a ground station, the spacecraft orbiting around a planet.
Alternatively, the support structure and the target are mounted on a first and a second spacecraft respectively, the first and the second spacecraft orbiting around a same planet.
Alternatively, the support structure is mounted on a ground station and the target is an orbiting spacecraft.